Process for producing bonded macroballoon structures and resulting product

ABSTRACT

A method of making light weight flame resistant structures from bonded polyimide macroballoons and products thereof. An aromatic tetracarboxylic acid dianhydride is reacted with an oxoimine to produce an N-substituted imide, which is then esterfied with a suitable alcohol. The resulting liquid is dried and the dry residue is reduced to a uniform powder having particles with diameters generally in the 0.5 to 10 mm. range. The powder is preferably further dried, either before or after final size reduction, in a moderate vacuum at moderate temperature to remove any excess residual alcohol. The powder spontaneously expands to form a closed cell foam when heated to a temperature in the range of about 90° to 150° C. for a suitable period. When the powder is expanded in a closed mold, a well consolidated, uniform, closed cell foam product results. The closed cell foam produced has excellent flexibility and resistance to heat and flame, and does not shrink appreciably when exposed to flame. When expanded in an unrestricted manner, closed cell &#34;macroballoons&#34; having average diameters between about 0.4 and 15 mm. result. The macroballoons are useful in a number of applications, including forming a foam-like structure by bonding them together with a very small amount of a suitable resin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of our copending U.S. patentapplication Ser. No. 423,802, filed Sept. 27, 1982.

BACKGROUND OF THE INVENTION

This invention relates in general to polyimide foams and, morespecifically, to closed cell polyimide foams having improved flameresistance.

A number of polyimide compositions having excellent flame resistance andutility as coatings and adhesives are described and claimed in U.S. Pat.No. 4,161,477, 4,183,138 and 4,183,139, granted to John Gagliani, one ofthe inventors of the invention claimed in this application.

The coating and adhesive compositions described in the above-mentionedprior patents are made by first preparing a suitable bisimide byreacting an aromatic tetracarboxylic acid dianhydride with a cyclicamide or oxoimine. The ratio of oxoimine to dianhydride is preferably inthe 2.3:1 to 2.7:1 range and the imidization reaction is preferablyconducted at a temperature of 170°-200° C. for 20-60 minutes.

The polyimide forming material is then prepared by dissolving thebisimide in an inert solvent; then adding thereto a suitable diamine,producing a viscous fluid containing an intimate, unpolymerized mixtureof N-substituted cyclic bisimide dicarboxylic acid and diamine which iscapable of being converted to a high molecular weight polymer by theapplication of heat.

The solution is coated onto a surface and polymerized by heating to atemperature in the 177°-316° C. range for 30 minutes to 5 hours. Thefollowing is exemplary of the exchange reaction which occurs: ##STR1##where n is a positive integer.

The resulting coating is tough, highly adherent to various surfaces,with very few pinholes or bubbles. It has excellent peel strength and isresistant to high temperatures, peeling and abrasion.

The prior coating material, however, was not suitable for use inapplications requiring a cellular or foam material, since conventionalagitation foaming and addition of known blowing agents add to processcosts and complexity and are not entirely effective at the relativelyhigh polymerization temperature required.

Co-pending U.S. patent application Ser. No. 390,778, filed June 21,1982, by the inventors of the present application, discloses and claimsmethods of making modified polyimide/polyimide-amide open cell foams andthe product thereof.

In that method, the reaction of an oxoimine with an aromatic dianhydridein a mole ratio of from about 0.05:1 to 1.5:1 produces an N-substitutedaliphatic monoimide which is then esterified with a suitable reactivesolvent. A suitable diamine is added and the liquid is dried to a filmor powder. The dry material spontaneously expands into a consolidatedopen cell foam when heated to a temperature in the range of about 230°to 320° C. At the lower end of this temperature range, a primarilypolyimide-amide foam results, while toward the high end of the range thefoam is primarily a polyimide. Flexibility, flame resistance and otherphysical characteristics vary with the proportions of the two polymersin the final product.

While this open cell foam is excellent for many applications, and canhave physical properties tailored for specific purposes, it is notoptimum for certain uses. The open cell foam tends to shrink whenexposed to flame or high heat. It may absorb water or other liquids andis permeable to liquids and gases. Also, the relatively high foamingtemperatures require more complex and expensive heating equipment andthe process is wasteful of energy.

Thus, there is a continuing need for a closed cell foam having acombination of flame and heat resistance, resistance to flame-inducedshrinking, flexibility and resistance to penetration by liquids andgases. Also, there is a need for reductions in process complexity andenergy consumption.

SUMMARY OF THE INVENTION

The above problems, and others, are overcome by a process having somesimilarities to those of the abovementioned patents and application,which, however, produces a flexible, high temperature and flameshrinkage resistant structure of bonded polyimide macroballoons. Whilethe reactions which occur are complex and not fully understood, itappears that under the polymerization reaction conditions describedbelow, the polymer produced is primarily a polyimide with a portion ofpolyimide-amide. For the purposes of this application the resin, forsimplicity, will be identified as a polyimide.

The basic steps in producing our polyimide macroballoons are reacting anoxoimine with an aromatic tetracarboxylic acid dianhydride in a moleratio of about 0.05:1 to 1.5:1 to produce an N-substituted imide;esterifying the imide with a reactive solvent; adding thereto a suitablediamine; drying the liquid to a nontacky, handleable material; reducingthe dry material to a powder having average particle diameters of fromabout 0.5 to 10 mm.; and heating the unrestrained powder to atemperature of about 90° to 150° C. for a suitable period to produce aplurality of discrete macroballoons by spontaneous foaming andpolymerization. Preferably, a step of further drying the material at amoderate temperature and a partial vacuum is performed just before orafter the powder size reduction step to remove any remaining excesssolvent.

DETAILED DESCRIPTION OF THE INVENTION

Either a consolidated, uniform closed cell shaped foam product or anumber of macroballoons or beads may be produced, depending on whetherfull foaming is restrained or unrestrained.

Macroballoons can be produced in diameters ranging from about 0.4 to 15mm. by unrestrained foaming, e.g., on a flat open surface or throughfeeding the powder into a flowing hot air stream. For bestmacroballoons, the powder size should be toward the smaller end of thepreferred 0.5 to 10 mm. general size range. The macroballoons haveutility per se, such as for packing material or fillers in other castingresins. As detailed below, a novel foam-like structure may be preparedby wetting the macroballoons with a small quantity, generally less than10 wt.% based on macroballoon weight, of a suitable liquid bondingagent, shaping the mass as desired, and curing the bonding agent at anysuitable temperature, preferably up to about 150° C. The foam-likestructures may be formed by any suitable method, such as by filling amold and curing the bonding agent, by continuously curing a layer of thewetted macroballoons to form panels or sheets, etc.

Since very little of the bonding agent is used, amounting to less thanabout 10 wt. % of the final product, the structures substantially retainthe high flame resistance of the polyimide macroballoons. The finalstructure will vary from rigid to fairly flexible depending upon thebonding agent selected.

Any suitable aromatic dianhydride may be used in the preparation of thedesired imides. Typical aromatic dianhydrides include those describedand referenced in the patents listed above. Due to their readyavailability at reasonable prices and the excellent foams which result,pyromellitic dianhydride and 3,3', 4,4' benzophenone tetracarboxylicacid dianhydride (BTDA) are preferred.

Any suitable oxoimine may be reacted with the selected dianhydride toproduce the desired imide. Preferably, the oxoimine has the generalformula: ##STR2## where "x" is a positive integer from 2 to 4. Of these,best results are apparently obtained with caprolactam.

While any suitable reaction conditions may be used, we have obtained thebest results where the dianhydride is added to the oxoimine and themixture is heated to about 150°-200° C. until imidization is complete,about 5-90 minutes. Optimum results have been obtained at about170°-180° C. for about 30 minutes.

In order to produce a superior foaming material, we have found that itis essential that the mole ratio of oxoimine to dianhydride be in therange of about 0.05:1 to 1.5:1. Above this range, the material forms acoating without foaming, while below this range excessively rigidmaterial is produced. Within this range optimum results occur with amole ratio of oxoimine to dianhydride of about 0.5 to 1.0.

The imides produced by the above reaction have the general formula:##STR3## wherein "x" is an integer from 2 to 4 and "A₂ " is selectedfrom the group consisting of: ##STR4## and mixtures thereof.

The imide thus produced is then esterified by dissolving it in asuitable reactive solvent at a suitable temperature. Any suitablereactive solvent which acts as an esterifying agent may be used. Typicalof these are aliphatic alcohols having up to 7 carbon atoms and aromaticalcohols, which may have halogen or amino substitutions, and mixturesthereof.

We have found that best results are obtained with alcohols havingboiling points between about 75° and 100° C. Lower boiling alcohols aretoo reactive and higher boiling alcohols require excessively highprocessing conditions to dry the resin. Optimum results are obtainedwith isopropyl alcohol, although good results are also obtained withethyl alcohol and, especially, secondary alcohols having low reactivityand boiling points in the above-mentioned range.

The esterification reaction takes place as follows: ##STR5## wherein "x"is an integer from 2 to 4, "A₂ " is as listed for the imide above and"R" is an aliphatic or aromatic radical which may have halogen or aminosubstitutions. This esterification may take place under any suitableconditions. Typically, a mole ratio of imide to esterifying agent offrom about 1:8 to 1:15 is preferred to assure rapid esterification atreflux temperature. This solution is heated to reflux (about 70°-90° C.)until clear, which takes about 60-90 minutes.

Once the esterification is complete, the selected diamine or diaminesare added to the solution. Preferably, an approximately stoichiometricquantity of diamine is used.

Any suitable diamine may be used. Typical diamines includemeta-phenylene diamine, para-phenylene diamine; 4,4'-diaminodiphenylether, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone,4,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl methane, 3,3'diaminodiphenyl methane and mixtures thereof. Of these, best results areobtained with 4,4'-diaminodiphenyl methane which is, therefore,preferred. If desired, aliphatic diamines may be used in combinationwith these aromatic diamines. Typical aliphatic diamines include 1,3diaminopropane, 1,4 diamino butane, 1,6-diaminohexane,1,8-diaminooctane, 1,12 diaminododecane and mixtures thereof.

Additives to improve various characteristics of the final foam may beadded as desired. Any appropriate additives may be used, such asfillers, surfactants and fire retardants to improve productcharacteristics.

Physical characteristics of panels fabricated from adhesively bondedpolyimide macroballoons may be varied by adding suitable quantities ofvarious fillers. For example, adding from about 1 to 50 weight percent(based on the final panel weight) glass microballoons to the panels willdecrease water absorption and increase the oxygen index. Best resultsare obtained with about 25 weight percent glass microballoons. Addingsuitable quantities of fibers such as glass, graphite, Kevlar polyamids,etc., will improve the load carrying capacity as well as the sheerstrength.

Microwave absorption of the polyimides can be modified and improved bythe addition of from about 1 to 50 weight percent (based on the weightof the final composite) prior to the drying step of any suitablemicrowave absorbing material such as graphite powder, ferrites,metal-ceramic compounds such as ferro titanate or mixtures thereof. Theoptumum combination of overall physical properties combined with maximummicrowave absorption material, based on the final composite structureweight.

Surfactants (surface active agents) are particularly desirable wheremacroballoons are produced, since they tend to produce higher cell wallthicknesses and walls of greater resiliency.

Typical surfactants include Dow Corning Corp. 190 or 193, FC430 fromMinnesota Mining & Manufacturing Co., Zonyl FSC from E. I. duPont deNemours & Co., and L550 from Union Carbide Corp. While any suitableconcentration may be used, from about 0.01 to 2% (by weight, based onthe weight of the solution prior to drying) is preferred. Of thesesurfactants, best results have been obtained with Zonyl FSC.

Fire retardants can be added to lower smoke emission, reduce flamespread and extend the burnthrough resistance of the products. Typicalfire retardants, which may be typically used in amounts of about 2 to 50wt. %, based on the weight of the final foam or macroballoons, includeFirebrake ZB, aluminum trihydrate, antimony oxide and mixtures thereof.

The solution is then dried by any suitable method. Simply heating thesolution in an oven to a temperature of about 65°-95° C. until dry issatisfactory. Other conventional methods, such as spray drying, rotarydrying, thin film evaporation, etc. may be used as desired. Theresulting free-flowing powder or flakes are non-tacky and may be storedindefinitely at room temperature.

We have found that in order to produce uniform high qualitymacroballoons, it is essential that the foaming powder be in the 0.5 to10 mm. average particle diameter range. Best results are obtained withparticle diameters in the 1.0 to 5.0 mm. range. If the drying processdid not produce particles in the desired range, the powder should beground or pulverized to produce the desired particle size. Typically,the powder may be reduced in size by mechanical grinding.

The resulting free flowing powder is suitable for the production ofclosed cell foam or macroballoons. However, for best results withgreatest uniformity, it is often preferred that the powder be furtherdried (either before or after reduction to the final particle size) at amoderate temperature in a moderate vacuum. Best results are obtained ifthe powder is dried for about 30 to 120 minutes at about 50° to 80° C.at a pressure of about 25 to 29 inches of mercury.

The final step in converting the powder into a foam or macroballoons isaccomplished by heating the powder to the selected foaming temperaturefor a suitable period.

The reaction which takes place is quite complex, since it is a combinedcondensation and exchange reaction. The reactions which are believed totake place are detailed in our copending U.S. patent application Ser.No. 390,778, filed June 2, 1982. Heating the powder to a temperature ofabout 90° to 150° C. for about 10 to 60 minutes causes the powder toexpand approximately 50 times in volume, producing a uniform, resilientclosed cell foam or macroballoons which appears to be a mixture ofpolyimide and polyimide-amide polymers. When foamed in a closed mold,using about 2 to 3 volume percent (based on mold volume) powder, a wellconsolidated foam product results. When foamed without volume restraint,individual macroballoons and/or agglomerations of macroballoons result.The foam or volume of macroballoons produced is tough, resilient andwill not shrink appreciably or emit significant smoke or toxicby-products when exposed to open flame.

Any suitable liquid bonding agent may be used to bond macroballoonsprepared as described above into structures. Typical bonding agents aresynthetic resins, including the polyimide liquid foam precursor of thisapplication, polyimide coating resins as described in our copending U.S.patent application Ser. No. 423,801 filed Sept. 27, 1982, epoxies,polyesters, polyimides, polyvinyls, synthetic rubbers, fluorinated andchlorinated hydrocarbon resins, phenolics, urea and melamine resins andmixtures and copolymers thereof. While any suitable temperature may beused, in order to avoid any chance of degradation of the macroballoons,resins curing at temperatures not exceeding about 150° C. are preferred.Somewhat higher curing temperature resins may be used with care. Onlysufficient resin to fully wet the macroballoons need be used. Generally,this results in a final structure in which less than 10 percent byweight is resin. This is preferred to maintain the flame and hightemperature resistance of the polyimide macroballoons and to keep thestructure as light in weight as possible. Of course, the intersticesbetween macroballoons could be entirely filled with resin, if desired.Any suitable wetting agent may be used to increase the wettability ofthe macroballoons by the bonding agent. The macroballoons may bepre-treated with etchants, solvents, abrasives, etc., as desired toimprove adhesion to the bonding agent thereto.

Details of the invention will be further understood upon reference tothe following examples, which describe preferred embodiments of themethods and compositions of this invention. All parts and percentagesare by weight, unless otherwise indicated.

EXAMPLE I

About 515.5 g. (1.6 M) of 3,3',4,4'-benzophenonetetracarboxylic aciddianhydride (BTDA) and about 45.2 g. (0.4 M) caprolactam are placed in athree liter flask and heated to about 175° C. After about 30 minutes atthis temperature the mixture is cooled to about 70° C. and about 965 g.of isopropanol is added. This mixture is heated to reflux temperature(about 90° C.). Reflux is continued until the mixture appears clear,about 70 minutes. The mixture is cooled to just below about 70° C. andabout 317.12 g. (1.6 M) 4,4'-diaminodiphenyl methane is added. Thismixture is refluxed (at about 90° C. for about 15 minutes, then iscooled to room temperature and coated onto aluminum foil. The coating isdried overnight at about 82° C. The brittle product is crushed andplaced in a vacuum oven at about 82° C. for about 30 minutes at apressure of about 27 in. Hg. The powder is then further crushed to anaverage (and fairly uniform) particle size of about 3 mm. A mold,preheated to about 110° C. is opened, about 2.5 volume percent of thepowder is placed therein and the mold is closed. After about 30 minutesat this temperature, the mold is opened and a consolidated, closed cellfoam product is removed. The product is found to be resistant to flameand to not shrink appreciably when exposed to direct flame.

EXAMPLE II

The procedure of Example I is repeated four additional times, varyingonly the particle size of the powder heated in the mold. Where Example Iused an average particle diameter of about 3 mm., these additionalexperiments use average particle diameters of about: II(a) 0.1 mm.,II(b) 0.5 mm., II(c) 10 mm., and II(d) 15 mm. The foam produced in II(a)is a mixture of open and closed cell foam having poor physicalproperties. Examples II(b) and II(c) produce good closed cell foams,while II(d) produces foam with large, irregular closed cells giving anon-uniform foam with lower uniformity and resiliency. Thus, foamingpowder particle sizes in the 0.5 to 10 mm. range are preferred for goodclosed cell foam, with best results around the 3 mm. size of Example I.

EXAMPLE III

The experiment of Example I is repeated except that the vacuum dryingstep is omitted. The final foam is a mixture of open-cell andclosed-cell foams due to the excess of solvent present in the powder.

EXAMPLE IV

The procedures of Example I are repeated, except that the second dryingstep using a partial vacuum is performed after the final particle sizereduction, rather than before. The foam produced is substantiallyidentical to that produced in Example I.

EXAMPLE V

The procedures of Example I are repeated, except that in place ofisopropanol, the following solvents are used: III(a) ethanol (BoilingPoint 78.5° C.), III(b) aminoethyl alcohol (B.P. 172.2° C.), III(c)2-fluro ethanol (B.P. 103.35° C.), III(d) methyl alcohol (B.P. 42° C.),III(e) benzene (B.P. 80° C.0, and III(f) acetone (B.P. 56.5° C.).Excellent results are obtained with the alcohols having boiling pointsin the 75° to 110° C. range (Examples III(a) and III(c)). The higherboiling alcohol of Example III(b) produces poor results due todifficulty in adequately drying the powder. The lower boiling alcohol ofExample III(d) is too reactive and produces poor foam. The inertsolvents of Examples III(e) and III(f) do not act as esterifying agentsand fail to produce a foam.

EXAMPLE VI

Five samples are prepared as described in Example I up to the heating tofoam step. The five powder samples are placed in preheated circulatingair ovens at the following temperatures for the following time periods:VI(a) about 125° C. for about 40 min., VI(b) about 210° C. for about 30min., VI(c) about 250° C. for about 50 min., VI(d) about 280° C. forabout 20 minutes and VI(e) about 380° C. for about 20 min. The samplesof Examples VI(b), VI(c) and VI(d) all produce foam having excellentphysical properties. The foam of Example VI(a) is not fully expanded andcured. The sample of Example VI(e) shows a mixture of open-cell andclosed-cell foams due to the high temperature of foaming.

EXAMPLE VII

The procedures of Example I are repeated four additional times, varyingonly the quantity of caprolactam used. Where Example I used about 45.2g. (0.4 M) of caprolactam to give a molar ratio of caprolactam to BDTA(515.5 g., 1.6 M) of about 0.25:1, these four additional experiments usecaprolactam quantities of about: VII(a) 180.8 g. (1.6 M, 1:1 ratio),VII(b) 220.8 g. (2.0 M, 1.25:1 ratio), VII(c) 271 g. (2.4 M, 1.5:1ratio), and VII(d) 361.6 g. (3.2 M, 2:1 ratio). The foam produced inexperiments VII(a) and VII(b) have excellent foam rise characteristics,while that produced in VII(c) has low foam rise and the sample of VII(d)does not foam. This demonstrates that ratios of oxoimine to dianhydridein the 0.05:1 to 1.5:1 range are necessary for the production of goodquality foam.

EXAMPLE VIII

The experiment of Example I is repeated, except that the followingdiamines are used in place of the 4,4'-diamino-diphenyl methane: VIII(a)m-phenylene diamine (0.375 M), VIII(b) 4,4'-diaminodiphenyl sulfone(0.375 M), VIII(c) 4,4'-diaminodiphenyl oxide (0.1875 M), and4,4'-diaminodiphenyl sulfide (0.1875 M). In each case the resulting foamhas a uniform cellular structure and has excellent heat and flameresistance. The flexibility and resiliency varies somewhat among thesub-examples.

EXAMPLE IX

The procedure of Example I is repeated with the only change being thesubstitution of the following oxoimines for the 0.4 caprolactamspecified in Example I: IX(a) 2-pyrrolidone (0.4 M), IX(b) 2-piperidone(0.4 M), IX(c) caprolactam (0.2 M) and 2-piperidone (0.2 M). The productin each case is an excellent flame resistant foam, with slight changesin physical properties with the different oxoimines.

EXAMPLE X

The experiment of Example I is repeated with four additional sampleswith the addition of the following agents to the liquid just prior tothe first drying step: X(a) about 0.5 wt. % (based on solids) Zonyl FSB(available from E. I. duPont deNemours), X(b) about 1.0 wt. % Corning190 (available from Dow Corning Corp.), X(c) about 43 g. of very finelydivided aluminum hydroxide and X(d) about 60 g. of about 0.25 inchgraphite fibers. The samples having the surfactants (Examples X(a) andX(b)) are found to have increased resiliency, while the filler ofExample X(c) produces somewhat brittle foams and the reinforcing fibersof Example X(d) results in greater foam strength.

EXAMPLE XI

The steps of Example I are repeated except for the drying steps. Theliquid is sprayed into a chamber preheated to about 75° C. using a highspeed atomizer. By carefully adjusting the atomizer, particles in thedesired size range are produced. Larger particles are ground to thedesired size range where necessary. Samples of the spray dried powderare foamed as described in Example I with and without the secondmoderate vacuum, drying step. While both samples produce good foam, thesecond drying is found to result in foam having slightly better physicalproperties.

EXAMPLE XII

Powder samples prepared by the procedures of Example I and Example XIare sprayed into a moving hot air stream maintained at about 300° C. Theparticles are found to spontaneously expand into macroballoons whichflow with the air stream to a collector.

EXAMPLE XIII

The experiments of Example XII are repeated, except that about 20 g. ofZonyl FSB surfactant is added to the liquid prior to drying. Themacroballoons obtained using the surfactant have slightly thicker wallsand greater resiliency.

EXAMPLE XIV

Macroballoons produced by the methods of Example XIII are mixed withjust enough of the liquid of Example I (at the pre-drying stage) to wetthe macroballoons. The mixture is packed into a mold preheated to about260° C. After about 30 minutes, the consolidated article is removed fromthe mold. The macroballoons are solidly bonded together by the curedresin. The liquid appears to have polymerized without significantadditional foaming.

EXAMPLE XV

A quantity of macroballoons produced in Example XIII are placed in amold which is slightly overfilled. The mold is closed, applying about0.5 to 1.0 psig on the macroballoons. The mold is maintained at about370° C. for about 15 minutes. A consolidated closed cell structureresults with the macroballoons adhering well to each other.

EXAMPLE XVI

The experiments of Example I are repeated with four additional sampleswith the addition of the following agents to the liquid just prior tothe initial drying step: XVI(a) about 5 wt. % aluminum trihydrate (basedon solids); XVI(b) about 30 wt. % aluminum trihydrate (based on solids);XVI(c) about 10 wt. % antimony trioxide; and XVI(d) about 15 wt. %Firebrake ZB. Processing is continued as described in Example I. Each ofthese foam samples, plus a sample of foam produced in Example I isexposed to a direct flame. Those samples of XVI which includes a fireretardant are found to produce less smoke, have slower flame spread andto have a greater resistance to burn through than the sample of ExampleI.

EXAMPLE XVII

Samples of foamable powder are prepared as described in Example I. Threemold set-ups are prepared as follows: two sheets of material (asspecified below) are temporarily adhered to rigid backing and arrangedin a facing arrangement, spaced about 0.25 inch apart with the foamablepowder uniformly spread over one sheet. The powder occupies about 5% ofthe volume between the spaced sheets. In these tests, in XVII(a) usestwo sheets of aluminum foil, XVII(b) uses two sheets of curedpolycarbonate resin and XVII(c) used one sheet of phenolic resin and onesheet of aluminum foil. In each case, the sheets and powder are heatedto about 120° C. for about 40 minutes, cooled to room temperature, andremoved from the rigid backing. In each case a well bonded compositesandwich of foam between the outer sheets results. The test of XVII(a)is repeated with the sheets pressed together to a thickness of about0.18 inch at a temperature of about 175° C., immediately after foaming.A higher density composite results.

EXAMPLE XVIII

Three aluminum honeycomb structures, each having a thickness of about0.5 inches, overall dimensions of about 12 inches by 12 inches andhoneycomb hexagon diameters of about 0.25 inch are provided. Onehoneycomb, XVIII(a) is partially filled (about 5% by volume) with afoamable powder prepared as described in Example I and two otherhoneycomb structures are entirely filled as follows: XVIII(b) withmacroballoons prepared as described in Example XII and XVIII(C) with themixture of macroballoons and liquid described in Example XIV. In eachcase, the honeycomb structure is held between two rigid mold surfacesand heated to about 250° C. for about 30 minutes. In each case, a strongstructure with the foam/macroballoons bonded to the honeycomb results.The sample at XVIII(c) shows superior bonding.

EXAMPLE XIX

A quantity of macroballoons is prepared as described in Example XIII.Weighed samples of the macroballoons are each mixed with just enough ofthe following catalyzed epoxy resins to thoroughly wet themacroballoons. Resins used are: XIX(a) Aralidite 7065 available fromCiba-Geigy, XIX(b) Epon 154, available from the Shell Chemical Co. andXIX(c) Aralidite 8047, a flame retardant epoxy from Ciba-Geigy. Thecoated samples are placed in open rectangular molds and the resins areallowed to fully cure at room temperature. In each case a rigidlightweight structure results. Reweighing the structures shows that lessthan 10 wt.% of the structures is resin. Each structure has excellentflame resistance with XIX(c) producing the greatest resistance.

EXAMPLE XX

Additional samples of macroballoons prepared as described in ExampleXIII are mixed with adhesive resin solutions as follows: XX(a) acellulose acetate butyrate adhesive, available from the Dow Chemical Co.under the Dow 276-V9 trademark, XX(b) a water dispersed polyvinylacetate emulsion available from the Borden Co. under the trademark GelvaS-55 and XX(c) another polyvinyl acetate emulsion available under theUcar Lates 130 mark from Union Carbide. The samples are placed in openrectangular molds and are heated to about 100° C. for about 60 minutes.Well bonded structures result with less than 10 wt.% resin.

EXAMPLE XXI

The experiment of Example I is repeated, except that the foamable powderis sprinkled lightly over a moving web instead of being placed in amold. The web is continuously moved through a heated zone where thepowder is heated to about 150° C. for about 10 minutes, causing thepowder to expand into macroballoons and a few aggregates of severalmacroballoons. Four samples of the resulting macroballoons are eachcoated with resin as follows: XXI(a) a polyamide available from GeneralMills Chemical under the Versamid C trademark and XXI(b) a polyamideavailable from the Dow Chemical Co. under the D.E.H. trademark, XXI(c) avinyl pyridine latex, Pliobond LVP-4668 available from Goodyear andXXI(d) VPX-500, a vinyl pyridine latex from E. I. duPont de Nemours. Thesamples are placed in cylindrical molds and any excess resin is allowedto drain away. The molds are heated as follows: XXI(a) 200° F. for about60 min., XXI(b) 300° F. for about 20 min., XXI(C) 140° F. for about 30min., and XXI(d) 150° F. for about 20 min. In each case, a well bondedlightweight structure results. Pre- and post-weighings show that lessthan about 10 of the structure weight is resin.

EXAMPLE XXII

Samples of macroballoons prepared as described in Example XXI are placedin two molds. The first mold is flooded with a mixture of aurea-formaldehyde resin (diluted to 40% solids, by weight) availablefrom American Cyanamid Co. under the Urac trademark and about 3 wt.%(based on resin weight) of a catalyst, HF6 from National Casein Corp.The second mold is flooded with a thin aqueous mixture of Resimene, amelamine-formaldehyde resin from Monsanto and about 4 wt.% (based onresin weight) of ammonium sulfate catalyst. Each mold is drained ofexcess solution. The first mold is heated to about 120° C. for about 2hours and the second mold is heated to about 90° C. for about 4 hours.The products are well consolidated lightweight structures, with slightlyover about 10 wt.% resin.

EXAMPLE XXIII

A quantity of macroballoons prepared as described in Example XIII isdivided into four samples. Each sample is mixed with enough catalyzedEpon 154 epoxy resin from Shell Chemical Co. to lightly coat themacroballoons. The samples are then treated as follows: XXIII(a) thecoated macroballoons are spread in an about 0.5 inch thick layer over athin aluminum foil, then a second foil is placed over the layer,XXIII(b) the coated macroballoons are spread in an about 1 inch thicklayer on a mold-release coated surface and a thin glass fiber fabricimpregnated with catalyzed Epon 154 resin is placed over themacroballoon layer and a second 1 inch layer of coated macroballoons islaid thereover, XXIII(c) an aluminum box section structural shape havinga length of about 2 feet and a 1×1 inch cross-section is filled with thecoated macroballoons and XXIII(d) a titanium honeycomb structure havinga thickness of about 2 inches and a cell diameter of about 0.3 inch isfilled with the coated macroballoons. Each of these structures is heatedto about 90° C. until cure is complete. In each case a lightweightstructure results with the macroballoons bonded to each other and to theadjacent elements. The panels of Examples XXIII(a) and (b) exhibitgreater strength and rigidity than macroballoon structures used alone.The structural members of Examples XXIII(c) and (d) show greater impactresistance and rigidity.

EXAMPLE XXIV

The experiment described in Example XIX(a) is repeated, with thefollowing changes: XXIV(a) about 5 wt.% (based on resin weight) offinely chopped graphite fibers is added to the epoxy coating liquidprior to coating the macroballoons, XXIV(b) about 10 wt.% (based onresin weight) of finely chopped glass fibers is added to the epoxycoating liquid just prior to coating the macroballoons. After coatingand curing as described in Example XIX(a), the lightweight structuresare tested and are found to be slightly stronger and more rigid than thecorresponding structures without the fiber additives.

Although specific components, proportions and conditions have beenspecified in the above examples, these may be varied with similarresults, where suitable. In addition, other materials may be added tothe foamable material, such as fillers, colorants, ultraviolet absorbersor the like.

Other applications, modifications and ramifications of the presentinvention will occur to those skilled in the art upon reading thepresent disclosure. These are intended to be included within the scopeof the invention as defined in the appended claims.

We claim:
 1. A process for producing high temperature and flameresistant macroballoon structures which comprises the steps of:reactingan oxoimine having the general formula:

    CH.sub.2 --(CH.sub.2)--NHCO

where "X" is a positive integer from 2 to 4 with an aromatictetracarboxylic acid dianhydride in a mole ratio thereof between about0.05:1 and 1.5:1 to produce an N-substituted imide; esterfying saidN-substituted imide by mixing therewith a reactive solvent; addingthereto at least one diamine; drying the resulting liquid composition;reducing the dried material to an average particle diameter of fromabout 0.5 to 10 mm.; separating said particles from each other; heatingsaid particles to a temperature in the range of about 90° to 150° C.while substantially preventing interparticle contact, whereby saidparticles expand producing a plurality of discrete macroballoons;coating said macroballoons with a thin layer of a liquid bonding agent;bringing said macroballoons into contact with each other in a desiredstructural configuration; and curing said bonding agent to a dry state;whereby a lightweight structure of bonded macroballoons results.
 2. Theprocess according to claim 1 wherein said liquid bonding agent is asynthetic resin selected from the group consisting of epoxies,polyesters, polyimides, polyvinyls, synthetic rubber, fluorinatedhydrocarbons, chlorinated hydrocarbons, phenolics, ureas, melamines andmixtures and copolymers thereof.
 3. The process according to claim 1wherein the bonding agent constitutes up to about 10 wt.% of the drylight weight structure.
 4. The process according to claim 1 wherein saidstructural configuration is obtained by filling a mold with said coatedmacroballoons and removing the lightweight structure from the mold aftercuring of the bonding agent.
 5. The process according to claim 1 whereinsaid desired structural configuration is obtained by curing said coatedmicroballoons in contact with a surface so that said bonding agent alsoserves to bond said macroballoons to said surface.
 6. The processaccording to claim 1 wherein said particles are heated and expanded intomacroballoons by feeding said particles into a moving hot air stream. 7.The process according to claim 1 wherein said aromatic tetracarboxylicacid dianhydride is selected from the group consisting of 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, pyromelliticdianhydride, and mixtures thereof, and wherein said oxoimine is selectedfrom the group consisting of caprolactam, 2-pyrrolidone, 2-piperidoneand mixtures thereof.
 8. The process according to claim 1 wherein saidoxoimine is caprolactam, said reactive solvent is isopropyl alcohol andsaid diamine is 4,4'-diaminodiphenyl methane.
 9. The process accordingto claim 1 further including the step of adding a filler to said liquidbonding agent prior to said coating step and wherein said filler isselected from the group consisting of aramid fibers, graphite fibers,glass fibers, carbon fibers, graphite powders, fluorocarbon powders andmixtures thereof.
 10. The process according to claim 1 further includingthe step of adding from about 2 to 30 weight percent, based on themacroballoon weight, of a fire retardant to said liquid compositionprior to said drying step; said fire retardant composition beingselected from the group consisting of aluminum trihydrate, antimonyoxide and mixtures thereof.
 11. The process according to claim 1 furtherincluding the step of adding prior to said drying step from about 1 to50 weight percent, based on final composite weight, of glassmacroballoons.
 12. The process according to claim 11 wherein about 25weight percent glass macroballoons is added.
 13. The process accordingto claim 1 further including the step of adding prior to said dryingstep from about 1 to 50 weight percent, based on final composite weight,of a radar absorbing material selected from the group consisting ofgraphite, ferrites and metal ceramic compounds.
 14. The processaccording to claim 13 wherein said microwave absorbing material is ferrotitanate, of which about 10 weight percent is added.
 15. The productprepared by the process of claim
 1. 16. The product according to claim15 wherein said liquid bonding agent is a synthetic resin selected fromthe group consisting of epoxies, polyesters, polyimides, polyvinyls,synthetic rubber, fluorinated hydrocarbons, chlorinated hydrocarbons,phenolics, ureas, melamines and mixtures and copolymers thereof.